Many diagnostic tests are carried out in the clinical field utilizing whole blood as a sample. These diagnostic tests often employ techniques that include separating the serum or plasma from the whole blood and using that serum or plasma as a test sample to obtain an accurate reading of blood analytes, such as glucose, cholesterol, potassium, etc.
Traditionally, plasma and serum have been separated from whole blood by centrifugation. However, centrifugation is time consuming and requires equipment that is not generally available outside the clinical laboratory. Accordingly, field testing of the numerous blood substances that require the separation of serum or plasma is difficult.
A number of devices have been devised to address this problem. These devices generally utilize filtering devices capable of various types of blood separation. Such filters have been implemented using paper, non-woven fabric, sheet-like filter material composed of powders or fibers, such as man-made fibers or glass fibers, and membrane filters having suitable pore sizes. Known diagnostic devices that employ such filters include U.S. Pat. No. 4,256,693, Kondo, et al., which discloses a number of filter materials used to test blood in a multi-layered integral chemical analysis device. U.S. Pat. No. 4,477,575, Vogel et al., describes a composition and process for permitting the separation of plasma or serum from whole blood utilizing glass fibers in combination with other absorbent layers. U.S. Pat. No. 4,753,776 to Hillman et al. describes a device which separates serum from the whole blood and, using capillary force, moves that serum to a separate compartment in the device to perform the diagnostic chemical reaction.
These prior-art devices, unfortunately, have proven to be impractical or unsuitable for certain field applications. The patents to Kondo et al. and Vogel et al., for example, are unsuitable in applications which, due to space and volume constraints, require a small separation filter. Other problems associated with these prior-art techniques, including the patent to Hillman et al., involve a requirement for an excessive amount of blood, inadequate air venting for an accurate diagnostic reading of the reaction, an inability to handle excess blood, and/or they typically require the operator of the device to time or measure the amount of blood that is applied. These problems significantly hamper the diagnostic testing process. In many instances, added steps of measuring introduce intolerable delays.